The Department of Energy has chosen supercritical water oxidation as a promising technique for destroying hazardous and radioactive mixed waste. Supercritical water oxidation (SCWO) occurs by mixing water with hazardous waste and then heating and pressurizing this mixed water to supercritical values in an extraction vessel or reactor. The benefits of SCWO processes include short residence times, nearly complete efficiencies for destruction, and effectiveness for a large number of organic compounds.
One of the other advantages of SCWO includes its use of water. Water is inexpensive, non-toxic, and easily separated from many products. Water is also a convenient solvent because the material to be processed may already be in an aqueous solution, thereby eliminating the need to remove water from the final product. Further, the polarity of water can be controlled by temperature and pressure. Water is thus a good solvent for polar materials.
Unfortunately, the critical parameters of temperature and pressure for water are much higher than for other solvents. These high temperatures and pressures, plus the presence of oxygen, corrode the surfaces of any metallic pans located in a SCWO reactor. Severe metal corrosion occurs when the hazardous wastes include halogens and nitrates.
Current solutions to preventing severe corrosion to the metallic pans located in SCWO reactors include lining or coating these parts with exotic metals. Examples of such exotic metals include nickel, palladium, and alloys consisting mainly of nickel and copper or of nickel and chromium. These metals are expensive, difficult to machine, and cannot be coiled easily to provide a compact reaction chamber. Further, these exotic metal liners last for only a few hours of operation before their corrosion requires replacement. Accordingly, SCWO reactors need a smooth, inert lining for their metallic pans to resist corrosion.
Other shortcomings of the prior art include maintenance of the integrity of the seals after repeated use. Design of the sealing arrangement for a high pressure vessel such as an SCWO reactor is important not only to extend the life of the seals but also to prevent loss of hazardous materials, which would result in violations of safety regulations. Related art commonly uses a lid or head with a series of bolts which apply pressure on a seal such as an O-ring or a gasket made of gold or silver. These seals are exposed to the same corrosive environment which destroys the metallic parts within these reactors. Additionally, the bolted-down lids of these reactors are labor intensive to open and close, do not lend themselves to automation, and create considerable down time which all combine to increase overall operating costs.
Examples of other related art include U.S. Pat. No. 5,147,597 by Roofthooft et al. Roofthooft et al. disclose a prestablized chromium protective film which retards the build-up of radioactive contaminants on the inside surface of light water reactor systems. The film has a thickness of at least 500 Angstroms. Roofthooft et al. concentrate on reducing the formation of a radioactive isotope, Cobalt 59, and the corrosive products which are created by Cobalt 59 and are retained by steel piping during normal use.
Other related art includes U.S. Pat. No. 5,135,709 by Andresen et al. Andresen et al. disclose a method for reducing corrosion exposed to high-temperature water such as steam or water at about 150.degree. C. or greater. The method concentrates on components formed from carbon steel, alloy steel, stainless steel, nickel-based alloys, and cobalt-based alloys. The method includes providing a reducing species, which can combine with the oxidizing species of the water, and forming a catalytic layer of a platinum group metal on the treated component. Andresen et al. do not specifically discuss high-temperature water containing hazardous wastes.